Why You’re Awesome: Part 1

I thought I'd finish 2008 with the first in what I intend to be a semi-regular series on some of the random cool stuff that's going on with your biology and how evolution got you there.

The first time single-celled organisms decided to switch to communism (i.e. stick together to become multi-cellular organisms like cabbages, hamsters and us), they immediately faced a compromise. Sure, sticking together meant more food and more safety, but reproduction was now a more complicated matter. And as life became more complex, reproduction just got slower and slower. And because the rate that we can evolve is strongly influenced by our reproduction rate, this means that the bacteria and other little blighters that like to infect us have a massive evolutionary edge. We measure the time it takes for organisms to reproduce in terms of generation times, the average time it takes between a new organism being born and it getting around to reproducing for itself. After millions of years of evolution, the differences between the generation times of single-celled organisms such as bacteria and us humans is massive. The average human takes some 25-30 years to reproduce. Bacteria can divide and then divide again in 20 minutes. And so, bacteria and viruses evolve at a simply astounding rate by comparison to us big old lumbering animal types. Although we’ve had immune systems since those earliest days of multi-cellular life, the mutation rate problem was still there. Our immune cells work by recognising patterns- bits of bacteria, the surfaces of viruses, maybe some of the proteins from a parasite. But what good is it having cells that see what’s on the outside of a bacterium when a bacterium can change its outfit a million times before your cells can get their pants on in the morning? Our immune cells had to work in terms of broad patterns, and if one of those patterns changed due to mutation, things invariably got ugly.

Of course, if that had been the end of the story, we wouldn’t be here to talk about it. There’d just be a load of bacteria hanging around wondering where all the easy food went. Evolution, in its elegant non-wisdom, provided a solution. The key was to fight fire with fire. We vertebrates evolved a special group of cells that, when first made by our bodies (and we make loads of them every day), undergo their own randomised mutations. These special mutations occur only within a set part of our genetic code- right in the part which recognises those nasty bugs and parasites. Just by allowing chaotic re-arrangements of that little part of our DNA in a select set of cells, we we’re suddenly back in the game. With enough of these cells, we can now recognise some 10e12 patterns (that’s one thousand billion). In effect, we harnessed the driving force of evolution itself. Tightly controlled chaos. And it worked. Every animal with a spine has those precious cells. The lymphocytes, also called T cells and B cells. They’re awesome really.

Flu virus image from the Centers for Disease Control and Prevention's Public Health Image Library (Public Domain). Lymphocyte image from the National Cancer Institute (Public Domain).

Horsey

Author's Note: This week The Biologista is doing science and also going to a sciencey conference. Hence, the re-run of one of his favourite posts from way back when. That being July. He will return later this week with a brand new post about Vaccination and Fear.

Current speculative thinking is that the immune systems of many individuals co-evolved in the presence of persistent parasitic infestation. The immune system therefore evolved to over-compensate along the anti-parasite axis. It needed to be able to deal with new parasitic infection on top of the un-clearable persistent infection. We call this parasite killing axis the Th2 response. We can imagine our immune system as a sort of see-saw, but with many sides. As one side, or axis of our immune response raises, it pushes the others down. Some infections require a broad, balanced response. Others need a strong, single axis attack. Our persistent parasites evolved, adapting to evade our Th2 response. Some would push our immune response along other axes, perhaps towards our anti-bacterial response. The upshot of this was that we required further Th2 over-compensation. This co-evolution has probably been ongoing since our pre-mammalian ancestors. This is evolution over a time on the order of hundreds of millions of years.

Suddenly, in the space of a mere 200 years, the western world eliminated normal everyday parasitic infection. From an evolutionary point of view, an advantageous trait had become redundant in the blink of an eye. Individuals who previously had an evolutionary edge suddenly had a disadvantage. The asthamtic, the hayfever sufferer and the general sneezy snot bag has an immune system that resembles a race horse suddenly lacking its burden. The jockey, that nasty little parasite, has fallen off. The horse is gleefully running for the finish line, thinking he is about to win.

This stands as a wonderful example of the importance of context in evolutionary traits and in the emergence of new mutations. Allergy is today seen as some sort of genetic "defect". In African countries where the parasitic trypanosome and schistosome problems are finally starting to be reduced, allergy is starting to emerge. Perhaps this is mere coincidence, but if our thinking is correct, allergy and asthma will increase considerably there over the coming years. That is of course assuming that the western world gets off its arse to help do something about health in Africa.

Horsey

Author's note: Analogy is wonderful thing. Let's do some analogy about allergy. Oh man I'm sorry. Really the main point is to do with how context is important in evolution.

Current speculative thinking is that the immune systems of many individuals co-evolved in the presence of persistent parasitic infestation. The immune system therefore evolved to over-compensate along the anti-parasite axis. It needed to be able to deal with new parasitic infection on top of the un-clearable persistent infection. We call this parasite killing axis the Th2 response. We can imagine our immune system as a sort of see-saw, but with many sides. As one side, or axis of our immune response raises, it pushes the others down. Some infections require a broad, balanced response. Others need a strong, single axis attack. Our persistent parasites evolved, adapting to evade our Th2 response. Some would push our immune response along other axes, perhaps towards our anti-bacterial response. The upshot of this was that we required further Th2 over-compensation. This co-evolution has probably been ongoing since our pre-mammalian ancestors. This is evolution over a time on the order of hundreds of millions of years.

Suddenly, in the space of a mere 200 years, the western world eliminated normal everyday parasitic infection. From an evolutionary point of view, an advantageous trait had become redundant in the blink of an eye. Individuals who previously had an evolutionary edge suddenly had a disadvantage. The asthamtic, the hayfever sufferer and the general sneezy snot bag has an immune system that resembles a race horse suddenly lacking its burden. The jockey, that nasty little parasite, has fallen off. The horse is gleefully running for the finish line, thinking he is about to win.

This stands as a wonderful example of the importance of context in evolutionary traits and in the emergence of new mutations. Allergy is today seen as some sort of genetic "defect". In African countries where the parasitic trypanosome and schistosome problems are finally starting to be reduced, allergy is starting to emerge. Perhaps this is mere coincidence, but if our thinking is correct, allergy and asthma will increase considerably there over the coming years. That is of course assuming that the western world gets off its arse to help do something about health in Africa.